The present invention relates to a plasma jet ignition system and more particularly to a plasma jet ignition system for an automotive internal combustion engine.
In order to extend the lean misfire limit of the conventional spark ignition internal combustion engines, there is a continuing interest in new ignition sources and their effects on engine performance and emissions. Various kinds of new ignition systems have been proposed.
As shown in FIG. 1, a plasma jet ignition system has been proposed wherein a plasma jet energy storage system is designed as an add-on system, which is used in conjunction with a conventional ignition system which provides the basic spark timing and high voltage trigger signal to the plasma jet ignition plugs.
Referring to FIG. 1, the conventional ignition system includes a spark energy storage system 5 which has a battery 1, an ignition coil 2 having a primary winding connected to the battery via a ballast resistor R.sub.B and a secondary winding connected to a distributor 4 via a high voltage diode, and an ignition module represented schematically by a breaker 3 connected to the ignition coil 2. The distributor 4 is connected to all of a plurality plasma jet ignition plugs 7 of the engine by a spark energy delivery harness which includes a plurality of spark energy delivery cables 20 each leading to one of the plurality of plasma jet ignition plugs 7.
The plasma jet energy storage system 6 includes a high voltage power supply 8, a charging resistor 9, a storage capacitor 10, a free wheeling diode 11 which improves the efficiency of energy delivery by preventing voltage reversal on the storage capacitor 10, and an inductor or a choke coil 12 which limits peak discharge current from the capacitor 10. The storage capacitor 10, free wheeling diode 11 and inductor 12 are arranged to form an energy storage and pulse shaping network. The energy storage and pulse shaping network is connected to all of the plasma jet ignition plugs 7 by a plasma jet energy delivery harness including a plurality of plasma jet energy delivery cables 19 each leading to one of the plasma jet ignition plugs 7. Steering diodes 13 are arranged to prevent the spark energy from flowing into the storage capacitor 10. Hence, a reduction in the spark energy which might have occured is prevented by the use of these steering diodes 13.
As illustrated in FIG. 1, the plasma jet ignition plug 7 has a first or rod shaped electrode 14, a second electrode 15 and an insulating body 16 which together with the first and second electrodes 14, 15 defines a substantially enclosed plasma cavity 17. The second electrode closes one end of the plasma cavity 17 and is formed with an orifice 18 therethrough. The first rod-shaped electrode 14 extends part-way towards the second electrode 15 whereby to define a plasma cavity gap between the first and second electrodes 14, 15. The first electrode 14 is connected to the distributor 4 through the spark energy delivery cable 20 and the plasma jet energy storage system 6 through the plasma jet energy delivery cable 19, while the second electrode 15 is grounded. When sufficiently high potential is applied across the first and second electrodes 14, 15, upon opening of the breaker 3, to cause electrical breakdown of the plasma cavity gap, the energy stored on the storage capacitor 10 is now dumped into the plasma cavity gap by the discharge current. With sufficient electrical energy being supplied to the plasma cavity 17 during a sufficiently short time period, a jet of plasma is produced. A portion of the plasma within the plasma cavity 17 is ejected out of the plasma cavity through the orifice 18.
As different from the conventional electronic ignition system, the plasma jet ignition system illustrated in FIG. 1 operates as follows: When a spark occurs between the first and second electrodes 14, 15, a plasma jet is generated within the plasma cavity 17. The electrically conductive state of the plasma cavity caused by the plasma induces the discharge of electric energy stored on the storage capacitor 10 in the form of a discharge current. This discharge current causes the gaseous area of plasma to extend. This plasma consists of free electrons and ions that are at a high temperature and are therefore highly energetic and chemically active. The plasma is produced by the shock heating of the gas confined in the plasma cavity 17 by the electrical energy. This raises the temperature of the confined gas and produces partial ionization of this gas. The sudden increase in temperature also raises the instantaneous pressure of the partially confined plasma, causing a portion of it to be ejected out of the plasma cavity 17. This high temperature and high energy (capacitor 10 equal to 0.25 .mu.F and charged to 3,000 V for a stored energy of 1.125 J) ejected gaseous flow causes the production of a great number of small-spot like flames within a combustion space, causing safe ignition of the air fuel mixture within the combustion space.
The plasma jet ignition system illustrated in FIG. 1 has a problem caused by the use of or addition of a plasma jet energy storage system in conjunction with a conventional electronic ignition system. The problem is in an increase in a capacity C.sub.s between the plasma jet energy delivery harness and the ground. This capacity C.sub.s is applied across or in parallel to the plasma cavity gap of each plasma jet plug 7. In order to produce a sufficiently strong spark across the plasma gap cavity as to induce a plasma within the plasma cavity 17, a relatively high voltage from 20 KV to 30 KV is required to be applied across the plasma gap cavity. However, if the capacity C.sub.s applied across the plasma gap cavity increases, a portion of the spark energy absorbed by this capacity C.sub.s increases as to cause the voltage across the plasma gap cavity to fail to reach the required high level, causing misfire.
Electromagnetic wave noise occurs because of transmission of high energy pulsation current through the plasma jet energy delivery harness. If shielded cables are used for the purpose of suppressing the wave noise, the quantity of capacity applied across the plasma cavity gap increases further, resulting in an increase in probability of misfire. Thus, the use of shielded cables is not practical and no practical proposal thus far has been made to suppress the wave noise.